The invention relates to an apparatus for the evaporation of liquids, especially monomers containing silicon, for the production of thin coatings containing silicon and/or silicon compounds by chemical vapor deposition (CVD) in a vacuum onto substrates, using a current adjusting device and an evaporator connected to the output of the latter.
In the fields of microelectronics, display technology and telemetering, thin coatings of SiO.sub.2, Si.sub.3 N.sub.4, a-C:H, and a-Si:C, among others, are employed. In the prior art the coatings are produced by the low-pressure CVD or plasma-enhanced CVD process. A great variety of gases are used as starting materials for these coatings; for example, SiH.sub.4 (silane) can be used. For a number of applications compounds are suitable which first must be converted to the gaseous phase, since they are liquid at room temperature and have a low vapor pressure. Examples are TEOS=tetraethoxysilane, or HMDS=hexamethyldisilane. In order to produce the said coatings repeatably with these liquids converted to the gaseous phase an absolutely constant, repeatable and controllable gas flow is necessary. For this purpose an inert carrier gas, Ar, He or N.sub.2 can be passed through the liquid compound in a known manner. The carrier gas becomes saturated with the compound by diffusion, so that it can then enter the reactor. The regulation needed for this purpose is accomplished by temperature control and carrier gas control. A disadvantage of this method is the ever present large amount of carrier gas which, of course, is also carried into the reactor and interferes with the process or reduces the process window. Moreover, a very precise and difficult temperature control is necessary, since the loading of the carrier gas depends directly on the temperature.
It is furthermore known to hold the liquid compound in a heated tank. The vapor over the fluid is likewise fed through a heated liquid controller into a reactor. The heating of the liquid controller is likewise very difficult.
An apparatus as well as a method of the kind described above is known (U.S. Pat. No. 4,947,789) which serves for the evaporation of monomers that are liquid at room temperature. For the production of thin coatings containing silicon and oxygen, chemical vapor deposition is used for this purpose. It is furthermore known to use a mass flow controller as the flow adjusting apparatus, wherein the monomer pumped by the latter is fed in liquid state to the evaporator. In this known method the reacted rate of flow is greatly limited, since at high flow rates of over 25 g/h a stable flow can no longer be produced. For this reason no reliable practice of the is possible by the known method.
The invention is addressed to creating an apparatus whereby perfect evaporation of the liquid is achieved in a simple and rapid manner.
The housing of the evaporator has an evaporator body whose surface is roughened, porous, or whose body is made at least partially permeable. In this manner the total surface area of the evaporator is increased in a simple manner and thus the time required for evaporating the liquid is substantially reduced. For this purpose it is advantageous that the surface area of the liquid to be evaporated is increased by atomizing it and thus the mass-flow rate at constant vapor pressure and a given temperature increases, and that the evaporator body is provided between the ultrasonic atomizer and the outlet opening of the housing and the ultrasonic atomizer is equipped with a diaphragm which is made to vibrate by means of an oscillator at medium frequency, or at another suitable frequency corresponding to the liquid. In this manner the liquid provided in a supply tank can be metered to the ultrasonic atomizer through a flow controlling system or metering device. The liquid stream which passes through the nozzle of the ultrasonic atomizer is broken up by the vibrations of the diaphragm into a great number of small droplets of the order of magnitude between 50 and 100 .mu.m. These droplets strike against the evaporator body provided in the housing of the evaporator so that a vaporization of the liquid is accomplished very quickly.
It is furthermore advantageous that the evaporator body divides the housing of the evaporator into an inlet space and an outlet space such that the liquid passes from the inlet space to the outlet space only through the evaporator body.
The evaporator body preferably occupies entire cross-sectional area of the housing of the evaporator.
The evaporator body can be made of a sintered material. In this manner a very great surface area is created in a simple manner without increasing the bulk of the evaporator body.
The evaporator body can be of a truncated pyramidal shape.
The housing of the evaporator body and/or the interior are preferably heated by means of a heating system which is in a working connection with a temperature control system.
The production of many small droplets is of great advantage, because this is a simple way to increase the surface area of the liquid. The evaporator body made of sintered metal makes the flow of liquid or vapor uniform.